Conventionally, polystyrene type resin foamed molded products obtained by packing the inside of a mold with polystyrene type resin pre-foamed particles and subsequently conducting heating and foaming are known to exhibit excellent rigidity, thermal insulation properties, lightweight properties, water resistance, and foam moldability. As a result, these foamed molded products are widely used as cushioning materials and thermal insulation materials for construction materials. However, these foamed molded products tend to suffer from poor levels of chemical resistance and impact resistance.
On the other hand, foamed molded products formed from polypropylene type resins are known to exhibit excellent chemical resistance and impact resistance. Accordingly, these foamed molded products are used in automotive components. However, because polypropylene type resins exhibit inferior retention of the foaming gas, the foam molding conditions must be controlled very precisely, which creates a problem of increased production costs. In addition, polypropylene type resin foamed molded products tend to exhibit inferior rigidity to polystyrene type resin foamed molded products.
In order to address these problems associated with polystyrene type resins and polypropylene type resins, foamed molded products containing a composite of a polystyrene type resin having favorable rigidity and foam moldability, and a polypropylene type resin having favorable chemical resistance and impact resistance have been proposed (for example, see Patent Documents 1 to 4).
Patent Document 1 has an object of providing a foamed molded product with practically applicable levels of impact resistance, rigidity and surface properties and the like, and discloses a method of obtaining pre-foamed particles formed from a modified polypropylene type resin composition obtained by heating an aqueous suspension containing 100 parts by weight of a polypropylene (a), 1 to 500 parts by weight of a vinyl type monomer component (b) per 100 parts by weight of the polypropylene (a), and 0.01 to 10 parts by weight of a radical polymerization initiator (c) per 100 parts by weight of the vinyl type monomer component (b), in some cases under conditions in which the vinyl type monomer component (b) undergoes no substantial homopolymerization, subsequently impregnating the polypropylene (a) with the vinyl type monomer component (b), and then further heating the aqueous suspension to at least a temperature at which the crystalline portion of the polypropylene (a) begins to melt substantially, thereby polymerizing the vinyl type monomer component (b).
Patent Document 2 discloses a method of obtaining spherical polypropylene type resin particles, the method including: dispersing 100 parts by weight of pellets or a powder of a propylene type resin containing from 1 to 10% by weight of an ethylene component in an aqueous suspension, adding approximately 30 to 150 parts by weight of a styrene type monomer to the suspension and heating at a temperature of 130° C. or higher, and then conducting a polymerization in the presence of a polymerization catalyst.
Moreover, depending on the intended application for the foamed molded product, it may be desirable to have the product colored black in some cases, and carbon is well known as a black colorant in such cases. However, it is also known that this carbon slows the polymerization of the styrene type monomer and can cause incomplete polymerization. In order to overcome this problem, methods have been proposed in which a polymerization initiator containing a polymerization initiator that generates tertiary alkoxy radicals as the main component is used to obtain a black styrene-modified polyethylene type resin foamed particle (for example, see Patent Documents 5 and 6).
In Patent Document 5, specific examples of the polymerization initiator include dicumyl peroxide, n-butyl-4,4-bis(t-butylperoxy)valerate, and 1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane.
Patent Document 6 discloses the combined use of a first polymerization initiator that generates tertiary alkoxy radicals and has a 10-hour half-life temperature of not more than 100° C., and a second polymerization initiator composed of 2,2-bis(t-butylperoxy)butane as the polymerization initiator. Specific examples of the first polymerization initiator include t-butylperoxy-2-ethylhexanoate, t-amylperoxy-2-ethylhexanoate, t-butylperoxy isobutyrate, 1,1,3,3-tetramethylbutylperoxy-2-ethylhexanoate, 1,1,3,3-tetramethylbutylperoxy-3,5,5-trimethylhexanoate, and di-t-butylperoxy hexahydroterephthalate.
Furthermore, the use of composite flame retardants containing a combination of a flame retardant and a flame retardant assistant to impart styrene type resins with self-extinguishing properties is already known. Examples of the flame retardant include tetrabromocyclooctane, hexabromocyclododecane, tetrabromobisphenol A diallyl ether, and tri(2,3-dibromopropyl) isocyanate. Further, examples of the flame retardant assistant include 2,3-dimethyl-2,3-diphenylbutane, 3,4-dimethyl-3,4-diphenylhexane, dicumyl peroxide, and cumene hydroperoxide. A multitude of styrene type resins that have been provided with self-extinguishing properties by using the above flame retardants and flame retardant assistants have been disclosed.
Patent Document 7 discloses a flame retardant thermoplastic resin composition in which a halogen-based flame retardant, a phosphorus-based flame retardant or an inorganic flame retardant or the like has been blended, as a flame retardant, into a styrene type resin.
Patent Document 8 discloses a self-extinguishing type foamable styrene type resin particle prepared by blending a styrene type resin particle with a composite flame retardant formed from the flame retardant tris(2,3-dibromopropyl) isocyanurate and a flame retardant assistant composed of 2,3-dimethyl-2,3-diphenylbutane or 3,4-dimethyl-3,4-diphenylhexane.
However, in the modified resin particles obtained using the methods disclosed in Patent Documents 1 and 2, ensuring that the polypropylene type resin exists in large amounts near the surface portion of the particles (namely, the polystyrene type resin exists largely within the central portion of the particles) has proven particularly difficult, meaning achieving satisfactory chemical resistance is problematic. Furthermore, a foamed molded product produced by conducting foam molding using the modified resin particles obtained via one of these methods tends to lack satisfactory heat resistance.
Furthermore, achieving satisfactory completion of the polymerization of a styrene type monomer is difficult even when a polymerization initiator disclosed in Patent Document 5 or 6 is used, and as a result, a foamed molded product having the targeted degree of blackness tends to be unattainable. In the case of applications such as members for use inside vehicles, the coloring of the product is particularly important, meaning a modified resin particle capable of providing an attractive foamed molded product has been keenly sought.
Moreover, in a styrene-modified polyethylene type resin foamed particle, if a large amount of polystyrene exists at the surface of the particle, then a satisfactory degree of blackness cannot be obtained even if carbon is added, and because polystyrene exists largely at the particle surface in the black styrene-modified polyethylene type resin foamed particles disclosed in Patent Documents 5 and 6, the degree of blackness tends to be inadequate.
Furthermore, even if the flame retardants and flame retardant assistants disclosed in Patent Documents 7 and 8 are impregnated into the foamed molded product disclosed in Patent Documents 1 to 4, a foamed molded product that exhibits self-extinguishing properties while retaining favorable levels of rigidity, foam moldability, chemical resistance, heat resistance and blackness can still not be realized.
[Patent Document 1]    Japanese Unexamined Patent Application, First Publication No. Hei 9-194623
[Patent Document 2]    Japanese Unexamined Patent Application, First Publication No. Sho 61-9432
[Patent Document 3]    Japanese Unexamined Patent Application, First Publication No. 2006-70202
[Patent Document 4]    Japanese Unexamined Patent Application, First Publication No. 2005-97555
[Patent Document 5]    Japanese Examined Patent Application, Second Publication No. Hei 5-54854
[Patent Document 6]    Japanese Unexamined Patent Application, First Publication No. 2006-111862
[Patent Document 7]    Japanese Unexamined Patent Application, First Publication No. Hei 9-255879
[Patent Document 8]    Japanese Unexamined Patent Application, First Publication No. 2004-211042